1. If you already have a Lv. 7 planet, it may be that you are tapping the gold coin button in the bottom right corner and so have already gathered all the coins.

To check for yourself: wait for your Lv. 7 planet to have finished production > Tap the gold coin button in the bottom right and check your Lv. 7 planet to see whether you have gathered its coins.

*The gold coin button in the bottom right allows you to gather coins from all your Lv. 7 planets with just one tap!

2. If you have recently adjusted your phone's time in the settings, it may be that you have already collected the coins produced for that time period. Please see "I accidentally changed my system's time settings. My planets aren't producing coins but they're hungry! What can I do?"